1. Field of the Invention
The present invention relates, in general, to a ranging apparatus and method for improving ranging performance in an Orthogonal Frequency-Division Multiple Access (OFDMA) system, and, more particularly, to a ranging apparatus and method for improving the function of detecting ranging codes and delay times in an OFDMA system.
2. Description of the Related Art
An Orthogonal Frequency-Division Multiple Access (OFDMA) system is a scheme for multiple user access which assigns sub-channels, each implemented by setting a group of sub-carriers as a single set, to respective users while using a larger number of sub-carriers than a conventional Orthogonal Frequency Division Multiple (OFDM) system (for example, IEEE 802.11a).
An OFDMA system requires the strict synchronization of a frequency with a symbol among multiple users who use a single OFDMA symbol while maintaining orthogonality between sub-carriers in the symbol. When the OFDMA symbol of one user is not synchronized with the OFDMA symbols of other users, the OFDMA symbol of that user acts as multiple access interference on the other users, and becomes a principal factor causing the degradation of the entire performance of a mobile communication system (for example, M-WiMAX, Wibro, etc.).
The above-described multiple access interference may occur even in the symbols of users who communicate with a base station after uplink synchronization has been achieved. However, there is a higher possibility that multiple access interference will occur in the symbols of users who perform an initial ranging procedure while communicating with the base station when uplink synchronization is not achieved yet. Therefore, in the multi-user environment of an OFDMA system in which multiple access interference occurs, a base station needs a method for preventing the degradation of the performance of a mobile communication system by detecting more accurate ranging codes and by estimating more accurate delay times for respective users during initial ranging procedures for the users.
A ranging method in a conventional mobile communication system is configured to perform Fast Fourier Transform (FFT) on signals received from a plurality of user terminals, extracting ranging channel signals from the FFT-transformed signals, compensating for linear phase components, and correlating resulting values with ranging codes. During this process, the maximum value of correlation result values is compared with a preset fixed threshold value, so that a ranging code is detected and a delay time is estimated. However, since this method is configured to compare correlation result values with the preset fixed threshold value, it is difficult to detect an accurate ranging code and estimate an accurate delay time when reception signals vary due to the influences of the characteristics of reception signals, multiple access interference, noise, etc. Therefore, such a method is problematic in that the degradation of the performance of a mobile communication system increases.